The invention relates to fluid-energy conversion devices in general, and particularly to a passive means for controlling the power output of a diffuser augmented turbine.
U.S. Pat. Nos. 4,075,500, issued to Oman et al and assigned to the present assignee; 4,132,499, issued to Igra; and 4,204,799 issued to de Geus are illustrative of patents teaching the use of boundary layer control slots in a diffuser augmented wind turbine. In the devices disclosed in each of the aformentioned patents, a diffuser for a wind driven turbine takes the form of a duct which channels the air driving the turbine. By flaring the duct outlet (i.e., making the duct outlet greater in cross sectional area than the duct inlet), a desired exit plane pressure is effected behind the turbine blades. Thus, a greater mass flow rate of air is caused to flow through the turbine's blades, and power output is increased accordingly.
To prevent separation or stagnation of the air along the exit stream boundary layer (next to the duct's inner surface) with respect to the air in the center of the exit stream, one or more openings are provided in the periphery of the duct around and/or behind the turbine blades. These boundary layer control slots serve to inject high velocity air from the external air stream and allow a shorter duct to be used to control the diffusion of the turbine wake.
It has been observed that a simple cyindrical duct increases the power output of a wind machine by about 41%. Using a diffuser with boundary layer control slots can yield a power augmentation factor of at least 4.25 over a similar unshrouded turbine, and factors as high as 5.5 may be achieved.
One drawback of the power augmentation realized in such systems is that high wind speeds may drive the turbine and generator beyond rated operating levels. It is thus desirable to provide means for limiting the maximum power output of diffuser augmented machines.
One possible means for limiting the power output of a wind turbine is suggested in U.S. Pat. No. 3,944,840, issued to Troll. By using a windspeed sensor to control the cross sectional area of entrance and exit openings to the turbine, the Troll device achieves a constant generator speed.
However, relatively large surface fore and aft of the turbine must be displaced in order to directly control the flow impinging on the rotor.
Another approach is shown in French Pat. No. 559,239, issued to Moigne. In the Moigne system, a series of vents, similar to the boundary control slots of the Oman et al, Igra and de Geus diffusers, are provided with doors which are mechanically connected to the turbine itself.
Neither Troll nor Moigne suggest, however, a wind turbine control which operates without need of mechanical or electrical linkages.